1. Field of the Invention
The invention relates generally to a control apparatus and method for an internal combustion engine. More specifically, the invention relates to a control apparatus for an internal combustion engine, which includes a variable valve mechanism that is hydraulically actuated by the hydraulic fluid discharged from an oil pump driven by an engine, and another variable valve mechanism that is electrically actuated by an electric motor, and a control method thereof.
2. Description of the Related Art
Usually, an exhaust system of an engine is provided with a catalytic converter that removes specific elements contained in exhaust gas. As such catalytic converter, a three-way catalytic converter is widely used. The three-way catalytic converter oxidizes carbon monoxide (CO) and unburned carbon hydride (HC), and reduces nitrogen oxide (NOx) to convert them into carbon dioxide (CO2), water vapor (H2O), and nitrogen (N2). Carbon monoxide (CO), unburned carbon hydride (HC), and nitrogen oxide (NOx) are three specific elements, to be removed, contained in the exhaust gas.
When the temperature of a catalyst included in the three-way catalytic converter is low, the catalyst does not function properly. Accordingly, when the engine is started while it is cold, the temperature of the catalyst needs to be increased quickly. Otherwise, unstable combustion takes place, which makes it difficult to purify the exhaust gas containing a great amount of the three specific elements described above.
Japanese Patent Application Publication No. JP-A-05-215000 describes an engine warm-up promotion device that promotes warm-up of an engine based on the combustion state in the engine. The engine warm-up promotion device is used in an engine including a bypass passage through which intake air may bypass a throttle valve arranged in an intake system; and a bypass valve that is arranged in the bypass passage and that adjusts the engine speed when the engine is operating at no load and low speed. The engine warm-up promotion device includes valve overlap duration changing means for changing a valve overlap duration during which both an intake valve and an exhaust valve are open; ignition timing changing means for changing ignition timing; warm-up state detecting means for detecting an warm-up state of the engine; operating state detecting means for detecting an operating state of the engine; and control means. When a signal from the operating state detecting means indicates that the engine is operating at no load and low speed, in the case where a signal from the warm-up state detecting means indicates that the engine is cold, the control means increases the valve overlap duration and retards the ignition timing as compared to the case where the engine is warm.
With the described engine warm-up promotion device, if the valve overlap duration is increased, the amount of gas remaining in a cylinder increases and the combustion proceeds more slowly. In addition, if the ignition timing is retarded, the combustion takes place at a later time. Because the combustion takes place at the later time and proceeds more slowly, exhaust gas having a high-temperature is discharged through an exhaust port. The high-temperature exhaust gas is used to increase the temperatures of engine coolant flowing in a cylinder heard and a catalyst arranged in an exhaust system.
In many cases, a valve overlap duration is increased by advancing open timing of an intake valve and retarding close timing of an exhaust valve using a variable valve mechanism called a variable valve timing controller (hereinafter, simply referred to as a “VVT controller”). Usually, an oil control valve (hereinafter, simply referred to as an “OCV”), operated by hydraulic fluid discharged from an oil pump driven by an engine, is used as an actuator for the VVT controller. The VVT controller is set to perform control so that the valve overlap duration becomes shortest when the engine is at a standstill. Accordingly, even if the open timing of the intake valve attempts to be advanced using the VVT controller immediately after the engine is started while it is cold, the valve overlap duration cannot be increased sufficiently, because the hydraulic pressure of the hydraulic fluid is low (in addition, the viscosity of the hydraulic fluid is high).
Japanese Patent Application Publication No. JP-A-05-215000, however, does not mention such inconvenience. Accordingly, with the engine warm-up promoting device described in this publication, the valve overlap duration may not be increased sufficiently immediately after the engine is started while it is cold. As a result, the engine may not be warmed quickly.